Multiplexed analog-to-digital (A/D) converters have input pins for receiving multiple analog inputs, and provide digital outputs that represent the analog inputs. The digital outputs are fed into a microcontroller which uses the digital outputs to generate a microcontroller output that is a function of the analog inputs. Typically, the A/D converter and microcontroller are part of an industrial process control transmitter and the microcontroller output represents a process variable that is compensated for temperature, power supply voltages, etc., all of which are analog inputs.
Various types of malfunctions can occur in the circuitry. One malfunction that can occur is a short between adjacent input pins of the multiplexer. Another malfunction that can occur is a microcontroller or multiplexer failure that results in one of the digital selections bits for the multiplexer becoming stuck at either a high level (1) or a low level (0). Still another malfunction that can occur is an A/D converter failure that results in one of the digital output bits becoming stuck at either a high level (1) or a low level (0).
When A/D converter systems are used in Safety Instrumented Systems (SIS), there is a need to identify when such malfunctions occur and provide an error or diagnostic output. A high degree of confidence is needed in the reliability of the A/D converter. A diagnostic output from the microcontroller is needed so that a process control system supported by the A/D converter system is alerted to malfunctions.